Advanced wireless devices may have multiple radios that operate on the same, adjacent, or harmonic/sub-harmonic frequencies. The radios may provide access to networks such as wireless wide area network (WWAN), a wireless local area network (WLAN), a wireless personal area network (WPAN), Global Positioning System (GPS), Global Navigation Satellite System (GLONASS), etc. Some combinations of radios can cause co-existence issues due to interference between the respective frequencies. In particular, when one radio is actively transmitting at or close to the same frequency and at a same time that another radio is receiving, the transmitting radio can cause interference to (i.e., de-sense) the receiving radio. For example, same-band interference may occur between Bluetooth (WPAN) and 2.4 GHz WiFi (WLAN); adjacent band interference between WLAN and Long Term Evolution (LTE) band 7, 40, 41; harmonic/sub-harmonic interference may occur between 5.7 GHz ISM and 1.9 GHz Personal Communications Service (PCS); and an intermodulation issue may occur between 7xx MHz and a GPS receiver.
Analog interference cancellation (AIC) cancels interference between a transmitter radio and a receiver radio by matching gain and phase of a wireless coupling path signal and in a wired AIC path, as shown in FIG. 1, where dt is a transmitted signal from a transmitter (aggressor) radio 102, and hc is a coupling channel (wireless or wired coupling path signal) from the transmitter radio 102 to a receiver (victim) radio 104. AIC 106 attempts to cancel the impact of the coupling channel hc as reflected via the negative sign on the output of AIC 106.
Interference cancellation techniques are commonly used in wireless communication systems to improve performance where undesired transmit interference (i.e., local interference) couples into a co-located receiver. These techniques are generally based on a single antenna scenario where there is only one transmit antenna and one receive antenna at a single location. In this case, an analog interference cancellation (AIC) circuit in the receiver may be used to mitigate the undesired transmit interference by subtracting a filtered copy of the transmit interference (directly available from the co-located transmitter) in the receive path to cancel the undesired transmit interference. In many systems, multiple antennas are used to provide increased flexibility with M transmit antennas and N receive antennas. However, prior art multiple antenna systems require separate AIC circuits for each transmit-receive coupling path, a total of MxN paths. And, in other cases, separate coefficient controllers for generating coefficients to input to the separate AIC circuits are also required for each transmit-receive coupling path. Therefore, the motivation exists for more efficient analog interference cancellation techniques which require fewer than MxN AIC circuits and coefficient controllers.